This invention relates to the field of specific ion sensing and more particularly to sensors comprising a solid state membrane responsive to the activity of a specific ion in a process solution. Typical prior art sensors for this purpose are disclosed in Jerrold-Jones et al U.S. Pat. No. 3,354,069, Farren et al U.S. Pat. No. 3,607,710, Vanslette U.S. Pat. No. 3,708,411 and Ross et al Canadian Pat. No. 763,082. Sensors disclosed in these and other references generally provide membranes for measuring any of a number of specific ions, such as for instance sulfide, chloride, lead, silver, fluoride, and potassium. Glass membranes have been used for many years for detecting the presence of hydrogen ion.
In the usual application of prior art sensors a measuring electrode fitted with an appropriate solid state membrane (for instance a silver sulfide membrane wherein sulfide ion activity is to be measured) is immersed in a solution of unknown ion concentration together with a calomel or other reference electrode. Migration of the specific ion through the lattice structure of the solid state membrane creates a half-cell potential closely following the well known Nernst relation. Simultaneously an essentially constant half-cell potential occurs at the reference electrode, and both electrodes are connected to a high impedance electrometer. Within the measuring electrode there is usually an electrolyte solution such as for instance an aqueous saturated solution of KCL and AgC1 to provide a bridge between the solid state membrane and an output terminal wire. Alternatively there may be a solid connection between the membrane and the terminal wire as shown for instance in the above mentioned Vanslette patent. The voltage difference measured by the electrometer provides a measure of the concentration or activity of the ion being sensed.
In Ser. No. 235,116 there is disclosed a process controller wherein are employed a pair of identical electrodes each fitted with a solid state membrane to detect the presence of the same type of ion. One electrode is immersed in a process solution and the other is immersed in a reference solution. An electrical bridge is provided between the solutions, and the electrode output terminals are connected to the input side of a difference amplifier. The output of the difference amplifier regulates the addition of a concentration correcting fluid to the process solution being monitored.
For applications such as process controllers of the above described type it is necessary to produce specific ion sensors characterized by rugged construction, ease of assembly, long life, high sensitivity and freedom from internal electrolyte leakage. None of the known prior art sensors has been found to be entirely satisfactory in all of these respects.